Its shape is oddly asymmetric: the walls narrow as they run from south-west to north-east, giving the interior a wedge-like plan that does not correspond neatly to any obvious domestic or ecclesiastical function. The stonework itself is described as well-built, which suggests this was not a casual addition, though what purpose it served remains uncertain.

A cashel is a type of early medieval stone enclosure, typically circular or roughly oval, built to define and defend a settlement or farmstead. The structure at Ballyganner sits within one such enclosure, leaning into its interior wall face as though constructed after the cashel itself, or at least designed to make use of it as a rear boundary. The walls of the structure range in height from 0.3 to 0.8 metres and vary considerably in width, from 0.4 metres on the north side to a full metre on the south. To the east, traces of a souterrain are visible. Souterrains are underground stone-lined passages or chambers associated with early medieval Irish settlements, used variously for storage, refuge, or ventilation. Their presence alongside a structure of uncertain form deepens the puzzle of what everyday life inside this enclosure may have looked like.